The present invention relates to a speed detector which is particularly suitable for digital processing purposes. The invention is particularly suitable for industrial sewing machines.
The industrial sewing machine is usually driven by a motor having an electromagnetically operated clutch and brake mechanism. Namely, the continuously rotating torque of the motor is coupled to the sewing machine shaft by friction when the clutch mechanism is operated for acceleration and decoupled to allow deceleration by the brake mechanism in order for the sewing speed to vary in a range from 200 rpm to 6000 rpm. To achieve this range of speed control it is necessary that the sewing machine speed be accurately detected.
The sewing speed can either be detected by generating a speed related voltage signal of which the amplitude represents the speed, a method known as amplitude modulation (AM) technique, or generating a speed related pulse signal of which the frequency represents the speed, known as frequency modulation (FM) technique. The present invention is based on the frequency modulation technique using a magnetic sensing coil.
Conventional sewing machine speed detectors, which have been most extensively used, comprise a ring-shaped, circumferentially magnetized permanent magnet which is driven synchronously with the sewing machine and a sensing coil for developing an AC voltage proportional to the speed of the sewing machine. However, this type of speed detector has a number of drawbacks in that it is difficult to manufacture permanent magnets with a high degree of accuracy sufficient to assure consistency in the magnitude of magnetization and in the degree of concentricity among the produced magnets. The desired consistency is necessary since such variations would result in errors in the speed signal. Further manufacturing difficult is the need to provide as many pole pieces as possible for high precision speed control purposes. The recent development of digital technology has made this technology economically applicable to many uses so that digital systems can be implemented at a lower cost than analog systems are. The digital implementation of sewing machine speed control requires that the speed pulse be exactly proportional in repetition frequency to the sewing machine speed. Moreover, the number of such pulses generated per revolution should be higher than 100, a value not attainable with conventional speed detectors.